The present invention relates to a carrier assembly for releasably holding a thin material. More particularly, the present invention relates to a wafer carrier assembly for use in chemical mechanical polishing/planarization of semiconductor wafers.
Semiconductor wafers are commonly constructed in layers, where a portion of a circuit is created on a first level and conductive vias are made to connect up to the next level of the circuit. After each layer of the circuit is etched on the wafer, an oxide layer is put down allowing the vias to pass through but covering the rest of the previous circuit level. Each layer of the circuit can create or add unevenness to the wafer that must be smoothed out before generating the next circuit layer.
Chemical mechanical planarization (CMP) techniques are used to planarize the raw wafer and each layer of circuitry added. Available CMP systems, commonly called wafer polishers, often use a rotating wafer carrier head that brings the wafer into contact with a polishing pad rotating in the plane of the wafer surface to be planarized. A chemical polishing agent or slurry containing microabrasives is applied to the polishing pad to polish the wafer. The wafer carrier head then presses the wafer against the rotating polishing pad and is rotated to polish and planarize the wafer. The mechanical force for polishing is derived from the rotating table speed and the downward force on the wafer carrier head. The chemical slurry is constantly transferred under the wafer carrier head. Rotation of the wafer carrier head helps in the slurry delivery as well in averaging the polishing rates across the substrate surface.
Another technique for performing CMP to obtain a more uniform polishing rate is the use of a linear polisher. Instead of a rotating pad, a moving belt is used to linearly move the pad across the wafer surface. The wafer is still rotated to average out the local variations. An example of a linear polisher is the TERES(trademark) polisher available from Lam Research Corporation of Fremont, Calif.
With either type of polisher (linear or rotary), the wafer carrier head is an important component of the polishing tool. The wafer carrier head provides means for holding and supporting the wafer, rotating the wafer, and transmitting the polishing force to engage the wafer against the pad. The wafer carrier head is coupled to a rotating mechanism that also applies a pressure to the wafer so that the wafer can rotate while being pressed against a polishing surface.
In conventional wafer carrier head designs, it is customary to employ the use of a wafer mounting pad or carrier film that is adhesively bonded to a wafer mounting plate. This film serves to absorb or conform to surface irregularities on the back side of the wafer and, due to its high coefficient of friction, prevent the wafer from rotating inside the wafer carrier head as the wafer carrier head is being rotated during the polishing process. However, these designs also require that the film be replaced following a set number of polishing cycles.
In designs that employ a wafer mounting plate and a wafer mounting pad or film, the wafer is held by the wafer carrier head via a series of holes in the mounting pad. The holes allow passage of vacuum forces to the side of the wafer that is in contact with the mounting pad. However, this design has the disadvantage of drawing polishing slurry back through the holes and up into the vacuum lines, necessitating a flush system to periodically flush out the slurry.
Other designs employ the use of an inflatable elastomeric membrane to hold the wafer as it is being transferred to a polishing surface. Once the wafer carrier head is lowered to a polishing surface, the membrane inflates and applies a downward force onto the wafer so that the wafer contacts the polishing surface. These designs also employ a fixed non-adjustable retaining ring. The carrier head is lowered so that the retaining ring contacts the polishing surface. The retaining ring then prevents the wafer from slipping out from under the carrier head as the membrane is being inflated so that the wafer contacts the polishing surface. However, this design has the disadvantage of requiring precise timing between contacting the ring to the polishing surface and inflating the membrane. If the membrane inflates before the retaining ring contacts the polishing surface, the wafer may extend beyond the retaining ring. The wafer will then lose its peripheral containment and will slip out from under the carrier head when it reacts to the frictional forced introduced by contacting the moving polishing surface.
Edge exclusion is another disadvantage of wafer carrier head designs that employ an inflatable membrane. Edge exclusion categorically is a portion of the wafer edge that does not receive the same degree of polishing action as the balance of the wafer. The result is a reduction of usable area for product production.
Wafer carrier heads should be capable of gimballing in order to accommodate changes in parallelism between the carrier head and the polishing surface. Many wafer carrier heads gimbal through the use of a mechanical gimbal. However, mechanical gimbals have the disadvantage of causing a moment arm to form whose length is equal to the distance between the mechanical gimbal point and the polishing surface. This moment arm in turn aggravates a problem known as xe2x80x9cdig inxe2x80x9d, a problem common to carrier heads that gimbal. Dig in occurs when the wafer mounting surface digs into the leading edge of the wafer and causes a higher removal rate at the wafer edge than the remainder of the wafer. The moment arm associated with mechanical gimbals multiplies this tendency, and the resultant xe2x80x9cdig inxe2x80x9d is directly proportionate to the length of the moment arm.
To alleviate the disadvantages of the prior art, a carrier assembly for releasably holding a wafer is provided herein. According to a first aspect of the invention, the carrier assembly includes a primary housing having an adjustable retaining ring that protrudes downwardly from the primary housing. A secondary housing, fixed to the primary housing, has a wafer holding mechanism positioned in an area surrounding the circumference of the retaining ring. The retaining ring is movable with respect to the primary housing. The retaining ring moves independently of the wafer holding mechanism, and retains an edge of the wafer on the polishing surface when the wafer is lowered onto the polishing surface.
In another aspect of the invention the carrier assembly includes a primary housing. The primary housing has an adjustable wafer retaining mechanism that is configured to retain an edge of a wafer on a polishing surface when the wafer is being lowered onto the polishing surface. A secondary housing is fixed to the primary housing and has an adjustable wafer holding mechanism. The wafer holding mechanism is configured to apply one of a downward force and an upward force to the wafer to retain and transport the wafer to and from the polishing surface and to retain the wafer on the polishing surface. The wafer holding mechanism provides an adjustable and controllable downward force on the wafer so that the wafer is uniformly polished when the wafer holding mechanism is retaining the wafer on the polishing surface.
In another aspect of the invention the carrier assembly includes a primary housing having a vertically adjustable wafer retaining mechanism. The wafer retaining mechanism retains the edge of a wafer on a polishing surface when the wafer is being lowered onto the polishing surface. A secondary housing is fixed to the primary housing and has a vertically adjustable wafer holding mechanism that retains and transports a wafer to and from a polishing surface and retains the wafer on the polishing surface. The wafer retaining mechanism and the wafer holding mechanism are configured to pivotally accommodate changes in parallelism between the wafer and the polishing surface when the wafer is being polished by the polishing surface.
According to another aspect of the invention, a method for handling a wafer to be polished includes the steps of receiving the wafer at a wafer carrier head so that the wafer contacts the wafer carrier head. A uniform upward force is applied from the wafer carrier head to retain the wafer. The wafer carrier head is then transported to a polishing surface, and a protruding retaining mechanism is lowered from the wafer carrier head onto the polishing surface so that the retaining mechanism contacts the polishing surface. A uniform downward force is then applied onto the wafer. The wafer retaining mechanism is then raised, and the wafer is polished.